This invention pertains to vertical storage and retrieval systems and, particularly, to controlling such systems to facilitate conveying any one of a plurality of vertically movable article storage carriers to an exact location at an operator's access station.
Vertically arranged storage and retrieval systems are used where it is desired to conserve floor space in a building. Typically, these storage systems are comprised of a vertically driven closed loop chain conveyor to which a plurality of material or article carriers are pivotally connected. The conveyor is driven to establish any of the carriers at a location or locations where articles can be inserted or withdrawn from the carriers.
Typical control systems for vertical storage equipment have four sensors adjustably mounted to the stationary frame of the equipment and a target mounted to the carriers. Thus, the sensors can detect the carriers as they pass on the conveyor. Proximity sensors are commonly used. The first sensor detects the carrier target when it is in close proximity to enable the locations of the carriers to be referenced to a single point. The second sensor counts the carriers as they move past it for system indexing. The third sensor senses the carriers as they pass in one direction and generates a signal that is used to interrupt power to the conveyor drive motor and to energize a brake that stops movement of the conveyor in that direction. The fourth sensor operates in a fashion similar to the third sensor, except that it senses the carriers and is used to slow and stop the conveyor when it is moving in the opposite direction.
One of the problems with prior art of vertical storage and retrieval (VSR) systems has been to get any selected carrier to stop exactly at the desired level at the operator access station. Inaccuracies result from tolerances or variations in the carrier support and drive mechanisms plus inability to anticipate accurately what the deceleration curve of the carries will be, especially when they are carrying markedly different loads. It is perceived that an underlying cause of poor stopping resolution in prior art systems is the one-to-one relationship that exists between the frame mounted sensors and the carriers being sensed.
A significant deficiency in prior art controllers is that they are dedicated to controlling a given conveyor system that is equipped with a specific number of carriers. Typically, the carriers are somewhat like open sided troughs that are compartmentalized along their length. In some applications, however, it is also desirable to subdivide one or more of the carriers with partition walls or shelves that create one or more compartments where one is above the other in a single carrier. Existing control systems do not have the capability of positioning any one of the intermediate vertically arranged compartments at a specific level at the operator access station. In some VSR installations it is desirable to have more than one access station, as, for instance on different floor levels or on opposite sides of the equipment. In this case, prior art equipment required duplicate sets of sensors and associated controls for each user access station. Separate controllers increased the cost of the equipment. Also, known control systems always require equal spacing or pitch between carriers on the conveyor which is a further limitation on their versatility. Moreover, in prior art systems the spacing of all of the sensors on the conveyor supporting frame must be individually adjusted to suit the particular carrier pitch even if the pitch or spacing between carriers is uniform.
In vertical storage equipment, the carriers are usually pivotally mounted to two parallel running closed loop conveyor chains. Thus, when the conveyor is being driven to establish a preselected carrier at the work station level, some of the carriers are running downwardly on one side of the chain loop and others are running upwardly on the other side. The weight of the articles in the carriers may differ so that a significant imbalance exists between the carriers on opposite sides of the conveyor. In prior art VSR systems, the accuracy with which a carrier can be positioned at the access station varies with the amount of imbalance that exists. Moreover, when there is significant imbalance, jerky starts and stops can result. The conventional solution to this problem is to interpose a speed reduction system between the drive motor and the carrier conveyor, but, this by itself, does not solve the problem of obtaining smooth starts and stops or significantly improved carrier positioning accuracy.